Load modulation refers generally to a systematic altering of a load impedance. In one instance load modulation may be employed to improve linearity in a circuit such as in a power amplifier, and in another instance load modulation may be utilized to signal back to a transmitting arrangement some piece of information. One application for such “signaling back” is wireless power transfer (WPT). One organization that has produced a specification for WPT systems is the Alliance for Wireless Power (A4WP). The A4WP specification (called “Rezence”) is designed to meet user experience and industrial design requirements for portable hand-held devices.
A basic wireless charging system may include a wireless power transmitter unit (PTU) and a wireless power receiving unit (PRU). For example, a PTU may include a transmit (Tx) coil, and a PRU may include receive (Rx) coil. Magnetic resonance wireless charging may employ a magnetic coupling between the Tx coil and the Rx coil. A common issue seen in these types of wireless charging systems is during an initialization phase. In an initialization phase, the PTU attempts to detect whether a valid PRU is being placed on or near the Tx coil of the PTU. For example, the PTU may be configured to sense load variations during a first predetermined beacon period to determine whether a device is on, or near the Tx coil. The load variations may be caused by a PRU being placed on or near the PTU, but may also be caused by a conductive metal of an object, such as a coin, or a device having a non-valid receiving coil in terms of a wireless charging protocol of the PTU.
For example, a PTU may power on during the first predetermined beacon period to detect whether a load associated with inductive coupling of the Tx coil by an object has changed. If a change in load is sensed, for example, compared to the previous period, the PTU will power on for a second predetermined beacon period that is relatively longer than the first predetermined beacon period. For example, the first predetermined beacon period may be 30 milliseconds. The first predetermined beacon period may be relatively short in comparison to a longer second beacon period of 100 milliseconds configured to follow the short beacon.